1. Field of the Invention
The present invention relates to a method and apparatus for interfacing a parallel connection.
2. Description of the Prior Art
When a digital communication system has to be established, in particular when synchronous communication systems have to be created, there is the need to implement short-distance connections, namely up to 300–600 m. For instance, said connections can be back plane or inter-shelf connections receiving a number of synchronous parallel flows at the inputs and multiplexing said flows on a certain number of output connections, which is generally lower than the number of input flows.
A good example is given by the SONET/SDH optical fibers networks, for which the Recommendation OIF (Optical Internetworking Forum) provides a standard for the short distance transmission for inter-shelf high-speed connections.
FIG. 1 illustrates an interface of the OC-192 VSR type.
Thus, FIG. 1 shows an interfacing apparatus 10 which receives at the input a data flow S64. The data flow comprises a STM-64 frame structure, namely a synchronous data flow at N bits, in particular where N is equal to 16. Therefore the input bit rate fi is 622.08 Mb/s. At the other side, the apparatus is connected to a M-bits optical connection O12, where in particular M is equal to 12. Therefore the optical connection operates at an output bit rate fo that is exactly twice the input bit rate fi (namely 1244.16 Mb/s).
The interfacing apparatus 10 is internally divided into two parallel paths, a transmission path 20 towards the optical connection O12, a receiving path 21 from the optical connection O12. In the direction towards the optical connection O12, the flow S64 finds firstly a parallel conversion element 11 carrying out the functions of synchronization of flow 64, namely of the STM-64 frame. The parallel conversion element further packets an information content CI into blocks of 240 bytes, synchronized with the frame itself. Still further, it adds parity bytes and error correction code bytes. Thus, M byte flows are obtained. In other words, the 16-bit word flow S64 which enters through the path 20, is re-allocated in packets P1 having M lines, one for each conductor/fiber that is available in the optical connection O12. Afterwards, the flow S64, converted to M bytes, is transmitted to a line encoder/serializer 12 which carries out the encoding of M flows and the conversion from parallel to serial. The so encoded and serialized flows are transmitted to a multiple electro-optical converter 13 which carries out the optical conversion for the output optical connection O12.
Vice-versa, from the optical connection O12 which enters the interfacing apparatus 10 on the reception path 21, the flows are first converted into an electric form through an electro-optical converter 14, then a line deserializer/decoder element 15 implements the M conversions of data from serial to parallel and the M independent decodings on M flows. Then, a parallel converter 16 carries out the depacketization by restoring the N bit frame structure and by performing flow delay compensation operations and error correction operations (no more than a single error for each block of 240 bytes). So, the synchronous N bit flow S64 is finally restored.
FIGS. 2a, 2b and 2c illustrate three occurrences of packets P1 of a type generated by the parallel link converter 11 according to the prior art: FIG. 2a illustrates the generic packet P1, FIG. 2b illustrates the packet P1 at the starting point of the flow S64, that is at the starting point of the frame structure STM-64, while FIG. 2c still illustrates the packet P1 at the starting point of the frame structure STM-64 after overwriting the necessary control bits that are requested for isolating the bytes of each single channel, for aligning the single flows one to each other and thus for restoring the frame structure of packets P1.
Each packet P1 is made-up of twelve lines because 12, that is the value of parameter M, is also the number of fibers of the optical connection O12, and of 24 columns, that is 24 bytes. The input frame structure contained in the flow S64, which is arranged on N-bit words, N being equal to 16, is aligned and therefore multiplexed on said lines or channels, namely it is inserted in a column bytewise manner in the first ten lines, which contain the information content CI or payload.
An eleventh parity byte line RP (XOR calculated on each one of the eight bits of the bytes of the respective column of information content, indicated in FIG. 2 by X1, X2, . . . X24) is added to these first ten lines. Said parity line RP is added to protect the operation of connection O12 also in the case of the single break of a fiber or fault of an electro/optical converter 13 or 14.
In addition, a twelfth line EDC is added consisting in 12 pairs of bytes: the first eleven pairs are the result of the calculation of an error detection code (CRC-16) that is applied, for each line, to the bytes of the 11 previous lines (indicated in FIG. 2 by CRC-1, CRC-2, . . . CRC-10 and CRC-P), while the twelfth pair contains the result of the calculation of the same error detection code when applied to the eleven pairs of previous bytes of the line EDC itself (indicated in FIG. 2 by CRC-C).
Said line EDC allows to identify the presence of errors in the packet P1 and, only in case of a single error, it is possible to correct it. In case of a single break of a fiber or of a fault of an electro/optical converter 13 or 14, no correction can be made.
Since the optical connection O12 is a parallel connection, each line or channel can reach the receiver downwards of the optical connection O12 (element 16 in FIG. 1) with a different delay. In order to provide the mutual re-alignment of channels and therefore to delimit the packets P1, each channel is codified by utilizing the known code 8B/10B and serialized in the element 12 of FIG. 1: this codification is usually employed in these cases, as it provides a signal with a high rate of transitions, continuously balanced and furthermore it allows to outline the bytes if some special control words are inserted at a certain frequency (so-called “comma”, typically the characters named K28.5). The problem is solved thanks to the known OIF technique by exploiting the fact that the transportable flow is only and solely a STM-64 frame (OC-192 in the SONET terminology): as already said, the STM-64 frame of 125 microseconds is aligned not only in order to extract the bytes to be inserted into the first 10 lines of the packet, but above all in order to align the flow of packets to the frame itself. The alignment of the STM-64 frame to the structure of packets P1 is possible as the number of bytes contained in the STM-64 frame (64*270*9=155520) is a multiple of the information content that is transportable by the packet (648*24*10=155520).
FIG. 2b illustrates the packet P1 at the starting point of the STM-64 frame: all the 192 A1 bytes and 48 A2 bytes are indicated as information content CI, which, in the SDH frames, are the bytes indicating the starting point of a frame.
FIG. 2c illustrates the same packet P1 at the starting point of the STM-64 frame, over which the first three columns with the codes K28.5, D3.1 and D21.2 have been overwritten: the codes K28.5, as already said, are used for selecting the bytes within each channel, as well as to select the channels (and therefore also the structures of the packets P1) one from each other, while the codes D3.1 (used in the first six channels) and D21.2 (used in the other six channels) are inserted to identify a possible wrong exchange of fibers during the installation of the optical connection (exchange of the first and the last fibers, of the second and the second-last fibers, and so on).
The need of inserting such frame designation bytes constitutes a trouble when, instead of a STM-64 data flow wherein it is possible to overwrite the first thirty bytes of the frame without troubles, there are other input signals with a different frame structure or also without any frame structure, as no overwriting of the payload, namely of the information content, is admissible.